In 1990, when the Journal of Functional Programming (JFP) was in the stages of being planned, I was asked by the then editors, Simon Peyton Jones and Philip Wadler, to contribute a regular column to be called Functional Pearls. The idea they had in mind was to emulate the very successful series of essays that Jon Bentley had written in the 1980s under the title “Programming Pearls” in the Communications of the ACM. Bentley wrote about his pearls:

Just as natural pearls grow from grains of sand that have irritated oysters, these programming pearls have grown from real problems that have irritated programmers. The programs are fun, and they teach important programming techniques and fundamental design principles.

I think the editors had asked me because I was interested in the specific task of taking a clear but inefficient functional program, a program that acted as a specification of the problem in hand, and using equational reasoning to calculate a more efficient one. One factor that stimulated growing interest in functional languages in the 1990s was that such languages were good for equational reasoning. Indeed, the functional language GOFER, invented by Mark Jones, captured this thought as an acronym. GOFER was one of the languages that contributed to the development of Haskell, the language on which this book is based. Equational reasoning dominates everything in this book.